Broadband High‐Energy Observations of the Superluminal Jet Source GRO J1655−40 during an Outburst

The X-ray/radio transient superluminal jet source GRO J1655-40 was recentlysuggested to contain a black hole from optical observations. Being a relativelyclose-by system (d \sim 3.2 kpc), it can likely provide us with richinformation about the physics operating in both galactic and extragalactic jetsources. We present the first simultaneous broad band high energy observationsof GRO J1655-40 during the 1995 July-August outburst by three instruments:ASCA, WATCH/GRANAT and BATSE/CGRO, in the energy band from 1 keV to 2 MeV. Ourobservations strengthen the interpretation that GRO J1655-40 contains a blackhole. We detected a two-component energy spectrum, commonly seen from othergalactic black hole binaries, but never detected from a neutron star system.Combining our results with the mass limits derived from optical radial velocityand orbital period measurements, we further constrain the mass of the centralobject to be between 3.3 and 5.8 M$_{\sun}$, above the well-established massupper limit of 3.2 M$_{\sun}$ for a neutron star (the optical mass function forGRO J1655-40 is 3.16$\pm$0.2 M$_{\sun}$). This system is therefore the firstgalactic superluminal jet source for which there is strong evidence that thesystem contains a stellar mass black hole. The inclination angle of the binarysystem is constrained to be between 76 and 87 degrees, consistent withestimates obtained from optical light curves and radio jet kinematics.Comment: 27 pages, 4 PostScript figures, Accepted for ApJ publicatio